Cannot Convert Adult Account to Child Account - Server Error

I had the same problem as described here:

Cannot Convert Adult Account to Child Acc… - Apple Community

and here:

Cannot Convert Adult Account to Child Acc… - Apple Community

Since I can't reply to these threads anymore, I'm posting a solution that worked for me here.

Like the others, I tried to change the date of birth for an apple id account (linked to me as a family organizer). The account had a date of birth which made it an adult account, but the actual date of birth made it a child account. During the procedure to correct the date of birth (i.e., login with the account to change, change the date of birth, get a confirmation from the organizer account) I faced the same server error when entering the (PayPal) confirmation code.

I tried this multiple times over multiple months without success. Then I called support and tried things with them (for hours) without success. In the end the support advisor told me that such a change is not possible. This left me in deep disbelief since there is a user interface for this. I almost lost my belief in Apple as my religion :)

Finally I figured out that my PayPal payment information still listed an old phone number (which I don't have access to anymore). For some weird reason the confirmation code was (correctly) sent to my current phone number.

To resolve this, I replaced PayPay payment with credit card payment and then tried the whole process for changing the date of birth again. This time it worked.

I hope this information is useful to others.